An adaptive echo canceller for two-wire, simultaneous two-way data communication at full bandwidth uses Nyquist-interval, rather than baud-interval, processing to achieve independence from timing discrepancies between near-end and far-end terminals. The entire echo signal, and not merely baud-interval samples thereof, is suppressed. The echo canceller is preferably a transversal structure.
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16. A method for use in conjunction with circuitry which transmits signals in response to near-end baseband data and which receives signals which include echoes of the transmitted signals, said method comprising the steps of
forming a plurality of samples of the received signals at greater than or substantially the Nyquist rate, said samples having respective components resulting from said echoes, forming a plurality of samples of an echo cancellation signal, the value of each one of l successive ones of said cancellation signal samples being a function of (a) a plurality of elements of said baseband data associated with said l samples and (b) a predetermined one of l sets of coefficients, l being a predetermined number, and combining each cancellation signal sample with a respective one of said received signal samples to form a plurality of compensated samples, the values of said coefficients being such that said compensated samples are substantially free of said echo components.
15. An arrangement for use in conjunction with circuitry which transmits signals in response to near-end baseband data and which receives signals which include echoes of the transmitted signals, said arrangement comprising
means for forming a plurality of samples of the received signals at greater than or substantially the Nyquist rate, said samples having respective components resulting from said echoes, means for forming a plurality of samples of an echo cancellation signal, the value of each one of l successive ones of said cancellation signal samples being a function of (a) a plurality of elements of said baseband data associated with said l samples and (b) a predetermined one of l sets of coefficients, l being a predetermined number, and means for combining each cancellation signal sample with a respective one of said received signal samples to form a plurality of compensated samples, the values of said coefficients being such that said compensated samples are substantially free of said echo components.
24. A method for use in an arrangement which transmits signals in response to near-end baseband data and which receives signals which include echoes of the transmitted signals, said method comprising the steps of
forming a plurality of samples of the received signals at greater than or substantially the Nyquist rate, said samples having respective components resulting from said echoes, forming a plurality of samples of a cancellation signal including the step of forming as each one of l successive cancellation signal samples the sum of the products of (a) the coefficients of a different one of l sets of coefficients with (b) respective signals each derived from a respective one of a plurality of elements of said baseband data associated with said l cancellation signal samples, l being a predetermined number, and combining each cancellation signal sample with a respective one of said received signal samples to form a plurality of compensated samples, the magnitude of each cancellation signal sample being substantially equal to the magnitude of the echo component of the respective received signal sample.
17. An arrangement for use in conjunction with circuitry which transmits signals in response to near-end baseband data and which receives signals which include echoes of the transmitted signals, said arrangement comprising
means for forming a plurality of samples of the received signal at greater than or substantially the Nyquist rate, said samples having respective components resulting from said echoes, signal processing means for forming a plurality of samples of a cancellation signal, said signal processing means including means for storing l sets of coefficients and means for forming as each one of l successive cancellation signal samples the sum of the products of (a ) the coefficients of a different one of the coefficient sets with (b) respective signals each derived from a respective one of a plurality of elements of said baseband data associated with said l cancellation signal samples, l being a predetermined number, and means for combining each cancellation signal sample with a respective one of said received signal samples to form a plurality of compensated samples, the magnitude of each cancellation signal sample being substantially equal to the magnitude of the echo component of the respective received signal sample.
23. A method for use in conjunction with data communications circuitry which accepts near-end baseband data having a predetermined baud rate, transmits signals representing said baseband data, and receives signals which include echoes of said transmitted signals, said method comprising the steps of
forming a succession of samples of said received signals at a rate which is greater than or substantially equal to twice the highest significant frequency in said received signals and which is l times greater than said baud rate, l being a predetermined number, forming a succession of echo cancellation samples combining each echo cancellation sample with a respective one of said received signal samples to form a succession of compensated samples, each one of l successive echo cancellation samples being equal to the sum of the products of (a) the coefficients of a different one of l sets of coefficients with (b) respective signals each derived from a respective one of a plurality of elements of said baseband data associated with said l samples, and repetitively updating the values of said coefficients in response to at least ones of said compensated samples in such a way as to minimize the energy in said compensated samples originating from said echoes.
6. In a two-way data transmission system having a four-wire to two-wire bridge between a common transmission link and each system terminal including separate transmitter and receiver sections,
a compensation circuit for transmitter signal components leaking across said bridge between transmitter and receiver sections at each terminal for forming a sampled echo cancellation signal, said compensation circuit storing a plurality of samples of digital data to be transmitted spaced by baud intervals and of zero-order samples at uniform intervening intervals no longer than the reciprocal of twice the highest frequency applied to said transmission link, sampling means for operating on incoming received signals at a rate greater than or substantially equal to twice the highest frequency employed on said transmission link to form a high-speed sampled sequence, and means for subtracting the sampled echo cancellation signal derived in said compensation circuit from the high-speed sampled sequence derived in said sampling means for forming an output signal substantially free of transmitter signal components for adaptive control of said compensation circuit, and recovery means for obtaining message data from applying the output of said substracting subtracting means to the receiver section of said each terminal.
14. An arrangement for use in conjunction with data communications circuitry which accepts near-end baseband data having a predetermined baud rate, transmits signals representing said baseband data, and receives signals which include echoes of said transmitted signals, said arrangement comprising,
means for forming a succession of samples of said received signals at a rate which is greater than or substantially equal to twice the highest significant frequency in said received signals and which is l times greater than said baud rate, l being a predetermined number, means for storing l sets of coefficients, means for forming a succession of echo cancellation samples and for combining each echo cancellation sample with a respective one of said received signal samples to form a succession of compensated samples, each one of l successive echo cancellation samples being equal to the sum of the products of (a) the coefficients of a different one of the l coefficient sets with (b) respective signals each derived from a respective one of a plurality of elements of said baseband data associated with said l samples, and means for repetitively updating the values of said coefficients in response to at least ones of said compensated samples such that the energy in said compensated samples originating from said echoes is minimized.
8. In combination with a digital data transmission system including terminals with transmitter and receiver sections for simultaneous two-way transmission at full bandwidth connected through a hybrid network to a common transmission channel comprising at each terminal
a data signal source in the transmitter section, an adjustable echo canceller having an input connected to said data signal source for an outgoing signal from the transmitter section, delay line taps spaced no further apart than the reciprocal of twice the highest frequency employed on said transmission channel and a summation circuit for selectively weighted signals on said taps for forming an echo cancellation signal, means for sampling incoming received signals at substantially or greater than twice the highest usable significant frequency on said common transmission channel to form a received digital sequence, means for subtractively combining said echo cancellation signal with said received digital sequence to form a compensated received signal, and means for applying said compensated received signal to said echo canceller for multiplication with outgoing digital data samples at the taps thereon for controlling the selective weighting of digital data samples at said taps, and means for recovering message data from applying said compensated received signal to the receiver section of said each terminal.
1. An echo cancellation arrangement for a baud-synchronous digital data transmission system comprised of terminals each having both a transmitter section and a receiver section for simultaneous two-way signaling at full bandwidth over a common signal path, said echo cancellation arrangement comprising at each such terminal,
means for sampling incoming received signals at a rate greater than or substantially equal to twice the highest frequency employed in said signal path, an adjustable signal processor for compensating for echoes of signals being transmitted by said transmitter section into said receiver section having an input connected to a data source in said transmitter section and an output combined in subtractive relationship with the output signal from said sampling means to form a subtractive output having an error component, said signal processor storing consecutive discrete-level samples from said data source at baud intervals and shifting such samples through a sequence of storage locations at intervals no greater than the reciprocal of twice the highest frequency employed in said signal path and such that an integral number of such shifting intervals occur in each baud interval, and means within said signal processor for computing the product of said consecutive samples with the error component of said subtractive output, and means for recovering digital data from applying the subtractive output of said signal processor,to said receiver section.
2. The arrangement defined in
a synchronously tapped delay medium, an adjustable gain device for each tap on said delay medium, means for entering a discrete-level digital data sample into said delay medium at baud intervals and zero-level samples at intervening times, tap-weight adjustment means for each tap on said delay medium under the control of the error component in said subtractive output, and means for combining tap signals operated on by said adjustable gain devices.
3. The arrangement defined in
4. The arrangement defined in
5. The arrangement defined in
first and second synchronously tapped delay media, an adjustable gain device for each tap on said first and second delay media, a fixed delay medium comparable in delay to the propagation time differential between near-end and far-end echoes in circuit with said first and second tapped media, means for entering discrete-level digital data samples into said first delay medium at baud intervals and zero-level signals at intervening times for further propagation through said fixed delay medium and said second tapped delay medium in tandem, tap-weight adjustment means for each tap on said first and second media controlled by the error component in said subtractive output, and means for combining tap signals operated on by said tap-weight adjustment means from both of said first and second tapped media.
7. The two-way transmission system defined in claim 5 6 further comprising at each terminal thereof,
a carrier wave source, a transmitter under the control of said carrier wave source for translating data signals to be transmitted to the passband of said transmission link, an upmodulator under the control of said carrier wave source for elevating the echo-cancellation signal from said compensation circuit to the passband of said transmission system, and a demodulator under the control of said carrier wave source in circuit between said subtracting means and said compensation circuit for translating the output signal from said combining means to the baseband frequency region.
9. The combination set forth in
10. The combination set forth in
11. The arrangement defined in claims, 1, 2, 3, 4 or 5 wherein said receiver section comprises means for recovering digital data from the subtractive output of said signal processor.
12. The two-way transmission system defined in
13. The combination set forth in
18. The invention of
19. The invention of
20. The invention of
21. The invention of
22. The invention of
25. The invention of
26. The invention of
27. The invention of
28. The invention of
29. The invention of
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This system restoring receivedincoming received passband signals to baseband in demodulator 59 the passband signals at the output of combiner 56. Demodulator 59 responds to the sinusoidal carrier wave from carrier source 58 to recover the original wave modulated onto a carrier wave at the distant data terminal. Upmodulator 65 is an apparatus responsive to a sinusoidal carrier wave from carrier source 58 to translate a baseband echo compensation signal from echo canceller 54 to the passband frequency level of incoming received signals prior to being subtracted from the passband received signal in combiner 56. Low-pass filter 60, baud sampler 61 and data sink 63 are substantially the same structurally as their counterparts 40, 41 and 43 in FIG. 2. Receiver 62 can comprise a demodulator for translating received signals to baseband frequency level. Receiver 62 is assumed to include its own demodulating carrier wave source.
The incoming received signal from transmission medium 75 traverses hybrid network 55 and is sampled at a rate no less than twice the highest significant frequency in the received signal as in the baseband embodiment of FIG. 2. The sampled output is in turn applied to one input of combiner 56, which has another input for accepting the echo cancellation signal. Since the echo cancellation signal generated in echo canceller 54 is in the baseband frequency region, it is necessary to translate it to the passband region of the received signal in upmodulator 65, which is under the control of carrier source 58. Carrier source 58 is also used to translate the outgoing transmitted signal to the passband of transmission medium 75.
The output of combiner 56 includes the sampled received signal from transmission medium 75 by way of hybrid network 55 compensated by an echo cancellation signal from echo canceller 54 upmodulated in modulator 65 to the passband region of transmission medium 75. Since the output of combiner 56 is at passband frequency, demodulator 59 under the control of carrier source 58 is provided to translate the compensated received signal, which is an error signal as far as adjustment of the echo canceller is concerned, back to the baseband frequency level. The demodulated error signal is applied to echo canceller 54 on lead 64.
The compensated received signal appearing on lead 66 is exactly analogous to the direct output of combiner 36 in the baseband embodiment of FIG. 2. It follows that low-pass filter 60, baud sampler 61, receiver 62, and data sink 63 are substantial counterparts of elements 40 through 43 in FIG. 2. The compensated received signal is thus conventionally detected to supply digital data to data sink 63.
FIG. 4 illustrates in simplified block diagrammatic form a sparsely filled transversal filter useful in the practice of this invention, specifically as an implementation of blocks 34 and 54 in respective FIGS. 2 and 3. Structurally, the transversal filter of FIG. 4 is conventional and comprises a plurality of delay units 80 connected in cascade so as to provide signal tapping points 81 at the beginning, intermediate points and end of a composite delay medium; a tap multiplier 85 at each tapping point 81; a tap-gain control unit 87 at each tapping point 81; a summation circuit 86; and an output terminal 90. One baud interval is assumed to span each four consecutive tapping points and successive tapping points are separated by the Nyquist interval Δ so that the ratio of baud to Nyquist interval is illustratively the integer four, i.e., l=4.
At the input terminal 12 On input lead 82 a sequence of baud-interval samples of the intended outgoing data sequence {an } is applied to the delay medium, illustratively a series of analog delay units 80 with individual delay amount Δ. Thus, at any instant of time only every fourth tap contains a non-zero signal sample (as is indicated by the notation an+1, an, and an-k) and the intermediate taps are unoccupied (as is indicated by the zeroes). Every tap has connected to it tap multiplier 85 and a tap-gain control unit 87. The tap-gain coefficients qn q-4, q-3, q-2, etc. appearing in the output of each tap-gain control unit 87 are derived from the product of a tap sample an-k and an attenuated error signal on lead 88 by an effective correlation process.
The effective correlation process is carried out in FIG. 4 by first multiplying the error signal generated directly in the output of combiner 36 in FIG. 2 or in the output of combiner 56 in FIG. 3 after demodulation in demodulator 59 by a step-size control factor β in multiplier 89 to form an attenuated error signal on lead 98. The factor β is preferably less than 1 and is subject to adjustment under the control of step-size control 91. A larger value of β may be used to advantage during a training sequence for establishing a connection between data terminals than during message transmission, for example.
Within each gain-control unit 87, as shown in detail at the tap 81 furnishing tap sample an-k, the attenuated error signal on lead 98 is multiplied by the tap sample in multiplier 92, whose output product is applied to a summer 93. The output of summer 93 is fed back to its input through a baud-interval delay unit 94 so that its instantaneous output is incrementally updated every baud interval in the manner of an integrator. The continually updated output of summer 93 is thus the tap-gain coefficient for tap multiplier 85.
Over a plurality of adjustments the overall effect in the tap weight applied by a tap multiplier 85 to a data sample at a tap 81 in FIG. 4 is analogous to what would result from a correlation of the error signal over the same period of time with the data signal. For this reason tap-gain control 87 can be loosely referred to as a correlator. Since the intermediate taps have zero-value samples, no contribution is made to the summed output of summation circuit 86 during each Nyquist interval by the intermediate taps. In this sense the delay medium is sparsely occupied. Nevertheless, the contents of the delay medium are shifted to the right each Nyquist interval by the interval Δ and a new set of tap-gain coefficients, qn act on the non-zero samples of the outgoing baseband data.
By way of example, at the time instant shown in FIG. 4 tap-gain coefficients q-4, q0, q+4, and so forth are active. In the next Nyquist intervak interval tap-gain coefficients q-3, q+1, q+5, and so forth are brought into use. Thus, the arrangement of FIG. 4 operates as though there were four differentially delayed delay media acting in parallel on the same input signal sequence. In this way an echo cancellation component is provided each Nyquist interval from baud-interval samples of the outgoing data sequence. As a practical matter the transversal structure can be limited to having taps at baud intervals with a sequence of tap-gain coefficients rotating at Nyquist intervals in a time shared arrangement.
Each multiplier 85 multiplies its associated tap sample an at a tap 81 by a tap coefficient qn to form a product of the form an qn . The summation of these products is taken in summation circuit 86 to form an echo cancellation signal on output terminal 90 according to the mathematic expression shown in FIG. 4.
It has been observed that, although the principal echo component appearing in the received signal is due to near-end local loop impedance discontinuities in, and leakage around, the hybrid junction, there also exists a far-end echo component from impedance irregularities at the telephone central office, at interfaces between sections of the transmission path (for example, at junctions between two and four wire links) and from the hybrid junction at the far-end terminal. The near-end and far-end echo groups are each dispersed over a few milliseconds. The magnitude of the dispersal is determinative of the number of taps required on the echo canceller. At the same time the interval between echo groups may be as much as 100 milliseconds on land circuits and up to 1000 milliseconds on satellite circuits. The distant echo although typically about 10 decibels below the near echo is nevertheless strong enough to degrade performance significantly. Rather than have an echo canceller spread over 1000 milliseconds, it is feasible within the principles of this invention to provide separate echo cancellers for each of the near-end and far-end echo groups and insert a bulk delay unit between the local data source and the echo-canceller which is assigned to operate on the distant echo group, or between active echo-canceller sections as shown in FIG. 5. The separate echo cancellation signals are first mixed to form a composite echo cancellation signal before being combined with the sampled received signal.
FIG. 5 shows an advantageous arrangement for generating in a single combined output echo cancelling signals for widely separated near-end and far-end echo components derived from the same transmitted data signal. The combination of a near-end and a far-end canceller comprises a near-end active echo canceller 101, a far-end active echo canceller 103 and a fixed bulk-delay unit 102. The data signal, whose echoes are to be compensated, appears on lead 12 108 and is applied to near-end canceller 101. This signal after propagating through canceller 101 is further delayed in bulk delay unit 102 before application to far-end canceller 103. Processing of samples of the data signal to be transmitted is identical in cancellers 101 and 103 under the control of an attenuated error signal on lead 98. Each of cancellers 101 and 103 is the same internally as that shown in FIG. 4. The two echo compensating components from cancellers 101 and 103 occur sequentially in time and are combined into a single compensation signal in summer 104.
The delay amount provided by bulk density delay 102 is determined by the length of the transmission path between data terminals. It is obvious that the input to bulk delay 102 can be connected to data lead 12 directly and have the same overall effect, provided only that the bulk delay include that inherent in near-end canceller 101. This alternate connection is shown as lead 105 in FIG. 5.
While this invention has been described in terms of specific illustrative embodiments, it will be understood that is it is susceptible to modification by those skilled in the art to which it relates within the spirit and scope of the appended claims.
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